1. Field of the Invention
The present invention relates to a servo control apparatus and to a method of stabilizing a servo control apparatus. More particularly, the invention relates to a servo control apparatus and a method of stabilizing a servo control apparatus for a feed drive system, such as in a machine tool, industrial machinery or robot.
2. Description of the Related Art
Feed drive systems using servo control, such as in machine tools, industrial machinery, or robots, generally include a speed loop which has a position loop as its main loop, and by which a speed controller of a servomotor is controlled so that the difference or deviation between a speed command value and a fed-back speed value becomes a zero.
FIG. 5 is a Bode diagram of speed control of a feed drive system for a machine tool. The Bode diagram plots a gain and a phase of the system for a range of frequency. In FIG. 5, there appear resonance and anti-resonance pairs near 100 Hz and 400 Hz, respectively. FIG. 4 is a Bode diagram for a speed control proportional gain xcfx89c=100 rad/s and a first-degree delay filter cutoff frequency xcfx89b=1200 rad/s. There is near 100 Hz an anti-resonance due to an axial rigidity of an associated feed drive mechanism, and near 400 Hz an anti-resonance due to a torsional rigidity of an associated ball screw.
In the above-described speed control system, if the gain is increased in order to improve response, there are cases in which instability occurs near a resonance peak frequency. For example, if the speed control proportional gain is increased from 100 rad/s to 400 rad/s, there occurs such a condition as illustrated in FIG. 6. In FIG. 6, resonance peaks near 100 Hz and 400 Hz both exceed 0 dB. However, near 400 Hz, the phase crosses 180xc2x0, and makes the servo system unstable.
For prevention of this condition, one approach that can be envisioned is that of using a low pass filter to reduce gains at the resonance peaks. The low pass filter is built in the speed controller, usually as a first-degree delay filter. By lowering the cutoff frequency of the first-degree delay filter, the gain can be reduced in a high frequency region. FIG. 7 is a Bode diagram for a case in which, by having an increased speed control proportional gain, the cutoff frequency is reduced from 1200 rad/s to 400 rad/s. In this case, although the resonance peak near 400 Hz is lowered below 0 dB, there is a reduction in phase margin near 100 Hz. This also leads to a tendency to instability of the servo system.
Another approach that can be taken is that of using a band stop filter to suppress the gain of resonance peak. This approach is illustrated in FIG. 8 to FIG. 10. FIG. 8 shows a characteristic of a control object (for the case having resonance and anti-resonance pairs at 100 Hz and 250 Hz, respectively), FIG. 9 shows a characteristic of a band stop filter, and FIG. 10 shows a synthesized characteristic in which the characteristics of the control object and the band stop filter are combined. In this case also, although the gain of resonance peak at the high-frequency side is suppressed so as to achieve stability, there is an accompanied increase in phase delay at the low-frequency side, with a tendency to go unstable near 142 Hz in the figure.
The present invention is made with such points in view. Accordingly, it is an object of the present invention to provide a servo control apparatus and a method of stabilizing a servo control apparatus, which enables the gain of a speed control system to be increased by suppressing the gain of a resonance peak to be stable, while possibly suppressing the increase of a phase delay in a low-frequency region.
A first aspect of the present invention to achieve the object is a servo control apparatus including a speed controller for controlling a servo motor so that a difference between a speed command signal and a speed feedback signal becomes a zero, wherein the speed controller has built therein a filter having characteristic that are reverses, or approximate the reverses, of at least one set of anti-resonance and resonance characteristics of a control object.
According to a second aspect of the present invention, the filter is an infinite impulse response filter having parameters to be set in accordance with anti-resonance and resonance frequencies and anti-resonance and resonance amplitudes.
According to a third aspect of the present invention, the servo control apparatus is used in a feed drive system of one of a machine tool, a piece of industrial machinery, and a robot.
A fourth aspect of the present invention to achieve the object is a method of stabilizing a servo control apparatus including a speed controller for controlling a servo motor so that a difference between a speed command signal and a speed feedback signal becomes a zero, wherein the method comprises the steps of having a filter built in the speed controller, and adjusting characteristics of the filter so as to cancel at least one set of anti-resonance and resonance characteristics of a control object.
According to a fifth aspect of the present invention, the method of stabilizing a servo control apparatus includes the steps of using as the filter an indefinite impulse filter, and setting parameters of the filter in accordance with anti-resonance and resonance frequencies and anti-resonance and resonance amplitudes.